Recirculating hydraulic installations are particularly adapted to the lubrication of automotive engines and transmissions. Lubrication serves not only to reduce friction, and thus wear, between the moving parts but also to disperse heat, to reduce corrosion and, in an engine, to assist in the sealing action of the piston rings.
In most modern engines, or transmissions, the lubricating fluid is stored in the pan, or sump, that normally comprises the lowest part of the crankcase, or transmission housing. The fluid is fed by a pump to the moving parts to be lubricated, and the fluid returns, by gravity, to the sump. In addition to serving as the reservoir, the sump also serves as a cooler because it is normally located in, or in proximity to, the air stream beneath the vehicle.
It must be appreciated that moving parts can lose considerable energy by virtue of parasitic drag resulting from high speed contact between the moving parts and the lubricating fluid. In addition, the turbulence created by such contact results in considerable entrainment of air in the hydraulic fluid. Aeration of hydraulic fluids such as lubricating oils, and particularly in the high temperature environment of an engine, or the like, allows a build up of acids in the lubricating fluid. Those acids are, of course, quite deleterious to the parts which the fluid is intended to lubricate.
In an attempt to obviate such difficulties many high performance vehicles employ a "dry sump" system for the engine and/or transmission. Dry sump systems store the lubricating fluid in an independent tank, or reservoir, which may, incidentally, also function as a cooling radiator. Prior known dry sump systems deliver the lubricating fluid from the reservoir to the parts to be lubricated by a first pump, and as that fluid collects in the sump it is generally scavenged from the sump by a second pump that returns the fluid to the reservoir in order to maintain the sump essentially dry.
The heretofore known dual pump arrangements have been quite successful in maintaining the sump relatively dry, but in doing so such arrangements have, unfortunately, fostered the entrainment of a considerable amount of air within the lubricating fluid. The undesirable entrainment of air results from unsuccessfully attempting to balance the rate, or volume, of the flow out of the sump (occasioned by the action of one pump) with the rate, or volume, of the flow returning to the sump (occasioned by the action of the other pump in delivering the fluid for the purposes of lubrication). This inherent difficulty with dual pump arrangements has been further compounded by the fact that in a moving environment, such as in a vehicle, the sump cannot be maintained in any one attitude. Thus, the normal motion of the vehicle tends to move the fluid toward and away from the intake to that pump which serves to scavenge the sump, thereby effecting the undesirable intrusion of air into the system.
In some environments it may be possible to provide a sump having sufficient depth so that the intake to the recirculating pump will remain submerged within the fluid in the sump during the normal movement of the vehicle over the roadway. However, the clearance between the roadway and the sump, and particularly in high performance vehicles, is not always sufficient to permit the use of a sump having the configuration necessary to assure that the intake for the recirculating pump will remain submerged.